MATERIALS OF THE EARTH’S CRUST
What is an element?
A substance which cannot be split into simple substances by physical or chemical means. Elements are made up of atoms. Atoms are made up of protons which are positive (+ve) charges, electron which have negative (-ve) charge and neutrons which do not have any charge
-Elements are about over 100 elements known so far
-About 90 elements exist naturally some elements exist naturally but they do not exist themselves they combine to form a compound (some exist independently which others are compound of more than one element.)
-Of the 90 element, 8 element are the most a bund and in the crystal rocks.
What is a mineral?
-Is composed of atoms arranged in a specific order which is 3 dimensional (crystalline) in structure.
-Are naturally occurring inorganic substance made up of elements or compounds which is 3 dimensional (crystalline) in structure.
Properties of Minerals
– Definite shape (crystalline)
– Is inorganic (naturally occurring substance)
– Minerals must be solid
– Made up of element or compound
Only minerals which are inorganic are minerals. Coal is not a mineral because it is not inorganic.
IDENTIFICATION OF MINERALS
In the field:
1) Its colour-Minerals have different colours
2) Luster -How a rock reflects the light. (Metallic luster they glitter and non-metallic luster are dull)
3) Streak -Powder which is obtained by rubbing the rock with a harder substance
For example: Iron will remove a darker streak than the colour of the iron .But if it is not a non-mineral, the streak will be lighter than the substance.
4) Cleavage -If break the rock it will split into definite shape (definite pattern) e.g. mica break into sheet pattern.
5) Crystalline – In 3 dimensional
6) Specific gravity- All mineral have higher specific gravity than water. H2O is 1g/cm3
7) Hardness -Minerals differ in hardness obtained through scratch test; there is a soft test & hardest test (Moh’s scale hardness; 10 minerals starting from softest of hardest.)
WHAT IS A ROCK?
– Is a more or less uniform mass made up of grains of one or more mineral which is found naturally on the earth crust.
– Are aggregates of minerals.
They can be classified according to :-
(a)Mode of formation
A. ROCK CLASSIFICATION ACCORDING TO MODE OF FORMATION
According to mode of formation/origin , rock can be classified as Igneous, Sedimentary and Metamorphic.
1. IGNEOUS ROCKS:
These are crustal rocks formed by cooling either within or outside of the earth’s crust. (Formed by cooling and solidification of molten materials from the interior of the earth)molten materials can solidify intrusively or extrusive (endogenically or exogenically)when molten material are still within the earth crust they are referred as magma, and when reach the surface they are called lava. Examples; basalt granite, Quartzite, Gabbro cools and solidifies to form volcanic rock
Igneous rock Is referred to as mother rock, due to the following reasons.
1) It is the rock from which other rock owes their origin. This means that other rocks are formed after some forces have operated on the igneous rock. Such forces are like weathering process which led to the formation of sedimentary rocks and metamorphic.
2) It forms the base for soil formation (pedogenesis) whose minerals composition, structure, texture and depth depend on the nature of the igneous rocks. E.g. soft igneous rocks, soil becomes deep and if a rock is hard the soil shallow.
3) The igneous rocks constitute the large proportion of the earth crust i.e.: 99%
CLASSIFICATION OF IGNEOUS ROCKS
1.) Extrusive rocks (volcanic) :
These are formed when the molten materials (lava) solidifies on the surface.
These rocks have small crystals because they cool fast due to exposure. Examples include
Basalt, Andesite, rhyolite, obsidian and etc.
2.) Intrusive rocks-
These are formed when the molten magma solidifies within the earth crust. They can be classified as hypabyssal igneous rocks, when they are formed near the earth’s surface and plutonic igneous rocks when they are formed deep inside the crust.
Categories of intrusive igneous rocks
(i) Hypabyssal igneous rocks-:
These are the rocks which are formed when magma cools and Solidifies inside but near the surface of the earth. They have medium size and examples are like granophyres, Porphyries, and dolerite. These can be exposed after erosion has taken place to form some rock masses like lopoliths and laccoliths which are usually hypabyssal. Basalt also quartz.
(ii) Plutonic igneous rocks:
These have solidified deep in the crust and they are seen on the surface only after being exposed by prolonged erosion. Example granite, gabbro, pumice and peridotite.
CLASSIFICATION OF IGNEOUS ROCKS BY THEIR CHEMICAL COMPOSITION
Felsic (acidic)igneous rocks:
Consist great amount of silica and feldspar with very little or no iron or any other metallic minerals examples are granite, granophyres, rhyolite, and obsidian. They are acidic in nature.
Mafic (basic) igneous rocks:
Consist of large amount of magnesium iron and other mineral like aluminium. They are basic in nature because of presence of metallic minerals e.g. gabbro, basalt
Consist of very large amount of metallic minerals like iron, magnesium and a little amount of silica less than 45% e.g. peridotite.
Intermediate Igneous rocks – With silica content between basic and acidic degree. That is, both acidic and basic oxides are in equal proportions. E.g. Diorite, Andesite.
CHARACTERISTICS OF IGNEOUS ROCKS
1) They are hard, they are semi precious.
2) They are formed by cooling and solidification of molten materials i.e. Magma or lava.
3) They differ in chemical composition depending on the amount of silica contained in them.
4) They are crystalline in nature (made of crystal)
5) They may undergo metamorphism to form metamorphic rocks.
6) They may undergo weathering process and sedimentation to form sedimentary rocks.
7) They contain minerals like iron, magnesium etc.
8) They don’t have fossils; therefore they do not contain the remains of skeletons’ of animals. Because they are made from the interior of the earth.
Are rocks formed from sediments deposited either by water or by wind or by ice. These rocks are formed by the process of the sedimentation. (Deposition of accumulation and lithification of some weathered particles and other materials). The sediment were laid down in layers or strata, one on top of the other and in time these layers turned into rocks as they became hardened by compression. These are called stratified rock.
-The plane between two layers is called the bedding plane.
-The angle of the titled strata to the horizontals called the dip
All sedimentary rocks are non-crystalline and many contain fossils. Some sedimentary rocks are formed in water .e.g. Inorganic-Sandstone and mud stone; Organic-Chalk, limestone and coral, and peat and coal (formed in swamps). Some are formed on land e.g. boulder cay, moraines and loess (all inorganic).
Those formed in water developed from inorganic sediments some sedimentary rocks are formed chemically and not from sediments.
There are three (3) types of sedimentary rocks;
1) Mechanically formed;
These are the rocks which have been formed by compaction and these are also referred as classic rocks because they consist of fragments of rocks and rocks materials. Examples; clay, gravels and alluviums (all deposited by water), moraines, boulder clay and gravels (deposited by ice), and loess (deposited by wind).
2) Organically formed;
These rocks have been formed by the accumulation, consolidation and cementation of the remains of died organisms. They include calcareous rocks like limestone and chalks formed from the shells and skeletons. Coral reefs are so common along the East African coast. These are also carbonaceous rocks formed from plant and remains buried a thousand of years ago under heat and the overlying pressure, the plant remains turned into rocks. Example Coal.
Also there are siliceous rocks formed as the result of the remains of organisms like
Diatoms and radiolarians, whose skeletons are rich in silica, Example diatomite rocks.
Example; Chalk and Coral (formed from animals), and peat, coal and lignite (formed from Plants). Rocks formed from organic sediments which are plants and animals remains which accumulate, compact and cement.
3)Chemically formed; these are the rocks which formed by chemical process. They include the following:-
a) Carbonates. Like travertine found in the form of stalagmites and stalactites due to the precipitation of calcium bicarbonate, trona due to the hydration of sodium carbonate solution after the evaporation of water.
b) Sulphate. As the result of desiccation and evaporation. Rocks like gypsum, which are hydrated calcium sulphate, may be formed as result of rapid evaporation.
c) Chlorides. Which include rocks salt is formed due to evaporation.
d) Silicates. Which include flint, sinter and cherty are formed due to the accumulation of silica followed by its compaction.
e) Iron stones. Are formed when iron oxide is deposited under a water body and compressed. Examples of such rocks include limonite and haematite (red ferric oxide).
|Mudstone||Bituminous coal||Rock salt|
CHARACTERISTICS OF SEDIMENTARY ROCKS
1. They are stratified and young rock layers overlying the old rock layers.
2. They are non- crystalline.
3. They contain fossils as the result of the accumulation of skeletons and shells of once living organisms.
4. They may undergo changes to form metamorphic rocks, when they are influenced by pressure and or temperature.
5. They consist of some fragments, which were deposited and then cemented to form the rocks.
6. They are soft (not hard).
3. METAMORPHIC ROCKS
These are rocks which are formed when one type of rock changes form after having been subjected to either intense heat, pressure or both. Any rock may undergo changes to form metamorphic rocks e.g.
Sedimentary rocks to metamorphic rocks are:-
1. Sandstone to Quartzite
2. Limestone to Marble
3. Coal to Graphite
4. Shale or Clay to Slate.
5. Mudstone to Slate.
Metamorphic to Metamorphic rocks
1. Slate to Schist
Igneous rock to metamorphic rock
1. Augite to hornblende
2. Granite to gneiss.
Causes of Metamorphism
1. Great heat -High temp. Acting on the existing rock. E.g. Bricks there from mud, also the intrusive igneous rocks can change to metamorphism.
2. Pressure- Rock can be compressed to form other rock resulting from earth movements.
3. Chemical reaction.
4. Pressure + Temperature can form sedimentary rocks to change to metamorphic.
Metamorphism The process of changing the rock from either Igneous, Sedimentary or Metamorphic rock to metamorphic rock.
Types of Metamorphism
1) Contact metamorphism -Rock change by contact, e.g. Intrusive magma will affect other rock and form metamorphic rocks Localized because it is not extensive it only affects the rocks which surround it.
2)Regional/ Dynamic Metamorphism– Result from pressure of internal movement which will compress the rocks to change their form. They cover a large area.
3) Thermal/dynamic Metamorphism – Combination of heat and pressure which can combine and change the form of the rock.
TYPES OF IGNEOUS ROCKS
|TYPE||FORMED AT||RATE OF COOLING||CRYSTALLINE NATURE||EXAMPLES|
|VOLCANIC||The surface||Fast||Small Crystals||Basalt|
TYPES OF METAMORPHIC ROCK
|Original rock||Metamorphic rock|
ROCK FORMING MINERALS
These are common minerals which make up large percentage of the rock of the earth’s crust.
These rocks are silicates.
1) Feldspar – Most abundant of all minerals in rocks. There are 2 varieties
i. Orthoclase feldspar of complex chemical formula KALSi3O8 a compound of Potassium, Aluminum, and Silicate.
ii. Plagioclause feldspar which contains sodium or calcium instead of potassium as in orthoclase.
: . you have NaALSi3O8 or CaAL2Si2O8 (plagioclase feldspar) e.g. Basic rock, basalt, gabbro, Feldspar have S.G (specific gravity) of 2.5g/cm3
2. Quartz-Second most abundant
– Chemical formulate (SiO2) Silicon dioxide
– Hardest common mineral
– S.G 2.7
– Has no cleavage
– Used for making concrete glass and as a semi precious stone.
3. Mica. Can be composed of many elements .e.g. aluminum, silicon, oxygen, iron, magnesium, hydrogen, or potassium.
-It is flat shiny rock most found in rocks like granite, gneiss or schist.
4. Carbonates: e.g. CaCo3 found in limestone and marble (Dolomite) Camg (co3)2
5. Horn blend: Common rock forming mineral dark crystalline mass associated with igneous +
– composed on calcium, iron and magnesium silicate.
6) Magnetite: (magnetic iron oxide) Fe3O4
Is the relationship in which rocks tend to change from one type of rock. For example, a rock can change from igneous rock to sedimentary or to metamorphic and then go back to igneous rock again.
Processes in the rock cycle
1. At first, the igneous rocks may be formed due to cooling and solidification of magma or lava.
2. Then igneous rock can be attacked by the weathering agents to form sediments which on being deposited and compacted may form sedimentary rocks.
3. Either igneous rocks or sedimentary rocks may undergo metamorphism due to the influence of either pressure or temperature or both to form metamorphic rocks.
4. Metamorphic rocks may undergo weathering and sedimentation to form sedimentary rocks.
5. Likewise, metamorphic rocks may undergo further metamorphism to form other more consolidated metamorphic rocks, like the change from slate to schist.
6. Lastly, rocks that are either sedimentary or metamorphic when subjected to very high temperature, they melt and on cooling form igneous rocks.
This process goes on repeatedly and is an endless process. No types of rock remain unchanged for a very long time.
IMPORTANCE OF ROCK
1. Rocks are important in the formation of soil which is essential for agriculture and plants growth.
2. Storing underground water. Water is stored in the water the water holding stratum of impermeable rocks and can come out as spring.
3. Some rocks are used as fuel like coal and mineral oil.
4. Rocks are also used for building and construction. Rocks like limestone, sandstone e.t.c are used for building houses and roads construction. Limestone is used for manufacturing of cement.
5. Salt extraction. Various Salts are obtained from rocks occurring in some places. E.g. in Tunisia, Morocco there is large deposited.
6. Manufacturing of chemicals, some rocks have salts such as nitrates or phosphates while others have potash. These Salts are used for making dyes fertilizers and medicines. Gypsum is used for making the plaster of Paris and Sand is used for making glass.
7. Mineral deposits. Mineral ores occur in veins of some rocks such as rocks. These ores were formed when magma cooled, valuable mineral extracted from rocks include gold, lead, copper, tin silver, zinc, aluminium calcium and manganese.
8. Some rocks are so impressive such that they attract some tourists to come and view them. In doing so the country gets foreign currency. E.g. Coral reefs.
9. Some rocks such as marble are used in decorating floors of some important buildings such as banks e.t.c.
How does metamorphism manifest (Shows) itself in rocks
1) By foliation – where by the rock has a distinct grain shown by the alignment of mineral particles, usually resembling stratifications but often in wavy lines.
2) By cleavage – Which resembles foliation although the direction of cleavage may be quit independent of stratification
3) By the development of new materials which were not present in the unaltered rock; precious stones and valuable ores may be produced by metamorphism.
4) By the development of a crystalline structure- in rocks which were originally amorphous rocks.
5) By the change of rocks which had originally a crystalline structure in to amorphous rock; e.g. marble (metamorphosed limestone and chalk). Slate metamorphosed shales.
B. CLASSIFICATION OF ROCKS BY GEOLOGICAL AGE
Rocks can also be classified according to age
I. Relative age.
II. Absolute age
1) RELATIVE AGE
Sedimentary rocks only where by the age of rock is done through the deposition.
a) Stratigraphy- stratigraphical sequences
– Arrangement of rock layers (Law of superposition). The rocks which are below are the oldest and the above are the youngest (the new ones)
Assume that the entire layer has deposit at the same time.
– This is only true where earth movement have not distorted the rock layers
b)Palaeontology -The study of earlier life as evident in sedimentary rocks. (Fossils)
– By studying with the earlier life brought about a GEOLOGICAL TIME SCALE
What is a geological time scale?
– Is a table dating in a chronological order the past events of the earth’s history?
Note: m y = million years
There a four ERA, Cainozoic, Mesozoic, Palaeozoic and Pre –Cambrian
Each era ends with a geological evolution which is seen in rocks and conformists recognized through different rock formation and change in type of life as evidence in fossils.
The eras are divided in periods and periods are subdivided into epoch and this is divided into series and formation.
In ERA,PERIOD and EPOCH process occur at some time and way across the world
Processes occurred before splitting of the continents started from series.
Each ERA varies from another there is variation in time where by pre –Cambrian takes 70% of the GTS.
Major geological event are affecting Africa continent mountains, building ,volcanism , glaciations and e.t.c
GTS gives relative age of rock but do not give the exact years of the rock.
2) ABSOLUTE AGE OF ROCKS
Modern method of determining age of rocks radio metric dating
This method is based on radioactive element which when they produce heat and another new substance which we call daughter element.
When the daughter element is produced the reaction of decaying stops.
The rate of decomposition disintegration from parent element two daughter element is constant
The new substance produced is used to calculate the age of the rock.
The time taken for parent element to disintegrate to form daughter element is called half life
–10 protons 82 protons
-22 neutrons 124 neutrons
– How is it used to find the age of rock.
It has been taken that 1gm of uranium yields 1 âˆ• 760,000,000 of lead a year.
. : A rock containing burn in it e.g. igneous rock and the weight of lead derived from it is found then age rock = (weight of lead âˆ• weight of uranium) x 7,600,000,000.
Example: lead – Uranium ratio in urinite crystal form pegmatite rock is 0.10
The age of that rock will be
0.10 x 7,600,000,000 = 760,000,000
Other elements used to determine the age of rocks.
|Isotope||Half life||Daughter element|
|1. K – 40||1.3 billion years||Ar – 40|
|2. U 238||4.5 billion years||Pb – 206|
|3. Rb 87||49 billion of years||Sr – 87|
|4. C14||5730 billions of years||N – 14|
– This method provide the absolute age of the rock.
– Give a critique on the geological time scale.
Importance of the Geological Time Scale
1. It depits the age of the rocks by showing the time when certain types of the rocks were formed and how were formed e.g. some were formed by glacial deposition while others were formed by volcanic eruption.
2. It also helps in understanding when and how different land forms were formed e.g. the mountains of different types like volcanic and Fold Mountains have been accounted for.
3. By studying the geological Time scale one is able to predict the occurrence of crystals deformation that are likely to take place e.g. by knowing that certain areas have old rocks one may conclude that faulting is likely to occurs in case any disturbance or stress.
4. Geological Time Scale also revel the life record for the plant and animals. This records help in understanding the relationship that exists between living things and the geological time scale process. Plants emerged when the soil had developed and animals came into existence when plants had already existed to provide food take on man emerged.
1.The methods that were used in determining the age of the rocks were largely based on estimate due to the limited power of the instruments which were used
2. Another problem pertaining to age determination has been caused by crustal deformation like over folding, gaps caused by denudation (unconformities) and the intrusions of magma.
3. There are some modifications made locally in the geological time chart. For example unlike the former geological time chart the current shows that Cainozoic era include the quaternary period. Also some other authorities do not indicate the Paleocene period in their geological time chart.
C. ROCK CLASSIFICATION ACCORDING TO STRUCTURE
It is the 3rd approach of rock classification.This involve the description of the rock.It may involve colour,size,hardness etc.However in order to classify rocks under this category,we use the term Permeability.
Permeability refers to the rate at which rocks can store water or the ability for water to pass through.Permeability can be divided into;
Primary Permeability or Porosity:
This involves rocks which have pore spaces.The size,alignment determine how much water can be absorbed.Porosity is greatest in Coarse-grained such as gravels,sands,sandstone and lowest in fine grained such as clays,granite.
When all pores are filled with water,the rock is saturated.Permeable rocks which store water are called Aquifers.
Secondary Permeability or Pervious:
These are rocks which have joints and fis-sures along which water can flow.The most pervious rocks are those whose joints have been widened.E.g
Carboniferous limestone o by cooling E.g Basalt.
Where rocks are porous or pervious,water,leaving the surface dry and without evident drainage e.g Chalk and Limestone regions.Impermeable rocks .E.g Granite,neither absorb nor allow it to pass through them.
Is the science which deals with development of land forms, (relief features).
MAJOR PROCESS WHICH SCULPTURE THE EARTHS CRUST
There are two major forces;
i. Endogenetic /internal processes
ii. Exogenetic /external processes
i. ENDOGENETIC PROCESS.
These are force which takes place beneath under the surface of the earth, and they are categorized as;
a. Earth movement:
ii. EXOGENETIC PROCESS
These are external force taking place on the earth surface.
These are force of gradation;
i. Destructive process(denudation) (Degradation)
ii. Aggradations (constructive process. (Deposition)